When organic wastewater is treated by activated sludge-process, a method of providing a settling tank behind a biological reaction tank, and of settlement-separating sludge and filtrate, is normal. However, since for such a method a large scale settling tank is required, in for example, sewage treatment plants in cities where sizes of site are necessarily limited, a membrane bioreactor process is being studied of arranging a separation membrane element in the interior of a reaction tank, of performing solid-liquid separation, and of taking out treated liquid as membrane-filtered water. This method makes a large-scale settling tank unnecessary.
However, in the membrane bioreactor process, a danger arises that sludge will deposits adherent to a surface of a separation membrane, and that within a short space of time the rate of a filtration through the membrane will deteriorate. Then, as shown in Patent Literature 1, a method has been provided of arranging an air-ejecting nozzle below a separation membrane element, of introducing a floating solid into a biological reaction tank, or causing the floating solid to flow by means of the lifting action of bubbles from the air-ejecting nozzle, and of scraping off the deposits adherent to the separation membrane surface. In the invention described in this Patent Literature 1, as the floating solid, a spongy foaming resin has been used.
However, in a conventional spongy floating solid, bubbles are gradually generated in communicating pores in the interior thereof, and, because, due to the hydrophobicity of the material used for the floating solid, it is difficult to remove the bubbles from spongy floating solid, spongy floating solid float on the upper part of a biological reaction tank, the effects of membrane-cleaning are easily lost. In addition, it is easy for the surface of the spongy floating solid to become gradually covered with microbes, or with ex vivo substances thereof, and since these are too soft, the effects of scraping off deposits (a scale layer) adherent to a membrane surface can not be obtained. Further, when a membrane-cleaning particle having few asperities is used on a surface of the floating solid, a problem has arisen that the scraping off of deposits (scale layer) adherent to a membrane surface has not been performed effectively.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 1997-136021